Hole-enhanced electron injection from ZnO in inverted polymer light-emitting diodes

Metal oxides as ZnO provide an interesting alternative for conventional low work function metals as electron injection layer in organic light-emitting diodes (OLEDs). However, for most state-of-the-art OLED materials the high work function of ZnO leads to a large injection barrier for electrons. As a result the electron current in the OLED is largely limited by the contact, leading to a strong reduction of the conversion efficiency. Here we demonstrate that by depositing an amorphous ZnO layer as cathode in an inverted polymer LED, the electron injection can be strongly enhanced by electrical conditioning. For suited polymers comparable conversion efficiencies of the conventional and inverted PLEDs can be achieved.